Point ahead offset angle for free space optical nodes

ABSTRACT

A free space optical (FSO) communication node communicates via an FSO link with a remote FSO communication node that moves relative to the FSO node. The FSO node may be highly directional, and transmit (Tx) and receive (Rx) beams of the FSO node may share optical paths (at least in part). Instead of directing a Tx beam along a point ahead angle relative to a Rx beam (which may result in undesirable Rx coupling losses), the Tx beam is directed based on the point ahead angle and a point ahead offset angle. The point ahead offset angle modifies the point ahead angle to reduce Rx coupling losses while keeping Tx pointing losses at least low enough to maintain the FSO link. In some cases, due to the point ahead offset angle, the Tx direction minimizes a sum of the Rx coupling losses and the Tx pointing losses.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Patent Application Ser. No. 63/021,025, “Point Ahead forIntersatellite Optical Links,” filed May 6, 2020, which is incorporatedherein by reference in its entirety.

BACKGROUND 1. Technical Field

This disclosure relates to communication between free space optical(FSO) communication nodes in motion relative to each other and, moreparticularly, to optimizing transmit and receive angles for the nodes.

2. Description of Related Art

FSO communication is an optical communication technology that uses lightpropagating in free space to wirelessly transmit data, for example, fortelecommunications or computer networking. Free space is a communicationmedium that can include air, outer space, or vacuum and contrasts withsolid communication mediums, such as optical fiber cables. FSOtechnology is useful where physical connections are impractical due tohigh costs or other considerations. FSO technology typically requiresvery accurate pointing between nodes to establish and maintain a viableFSO link.

SUMMARY

A free space optical (FSO) communication node (also referred to as aterminal) in motion relative to a remote FSO node may set a transmit(Tx) direction of a Tx beam towards an expected future location of aremote FSO node. This point ahead angle may cause a large misalignmentbetween the local FSO node and a receive (Rx) beam from the remote FSOnode. This may contribute substantially to Rx coupling losses since onlya portion of the Rx beam is collected by the local FSO node.

Embodiments described herein overcome this problem. Instead of directingthe Tx beam along a point ahead angle (which may result in undesirableRx coupling losses between the Rx beam and the FSO node), the FSO nodedirects the Tx beam in a direction based on the point ahead angle and apoint ahead offset angle (also referred to as a point ahead correction).The point ahead offset angle modifies the point ahead angle to reduce Rxcoupling losses while keeping Tx pointing losses at least low enough tomaintain the FSO communication link. In some cases, this modified pointahead angle minimizes a sum of the Rx coupling losses and the Txpointing losses. Thus, by directing the Tx beam based on the point aheadangle and point ahead offset angle, the Rx and Tx coupling efficienciesbetween the nodes may be increased without adding additional componentsto the FSO nodes.

Some embodiments relate to an FSO communication terminal includes a Txpathway, a Rx pathway, a beam steering unit, and a control system. TheFSO communication terminal may be collinear or co-borsighted. The Txpathway is configured to transmit a data-encoded Tx optical beam throughfree space to a remote FSO communication terminal. The Tx pathway isoriented along a direction in free space, and a relative motion betweenthe two FSO communication terminals is known. The Rx pathway isconfigured to receive a data-encoded Rx optical beam transmitted throughfree space from the remote FSO communication terminal. The Rx pathway isoriented along the same direction in free space as the Tx pathway. TheRx pathway may remain oriented along the same direction as the Txpathway during operation of the FSO communication terminal. The beamsteering unit is configured to adjust the direction of the Rx and Txpathways. The controller system is configured to apply a point aheadoffset to the direction of the Rx and Tx pathways based on both (a) anestimated reception of the Tx optical beam at the remote FSO terminal(given the known relative motion) and (b) an estimated reception of theRx optical beam at the FSO terminal (given the known relative motion).

The estimated reception of the Tx optical beam at the remote FSOterminal may be based on a Tx pointing loss function and the estimatedreception of the Rx optical beam at the FSO terminal may be based on aRx coupling loss function. In some embodiments, the point ahead offsetminimizes a sum of the Tx pointing loss function and the Rx couplingloss function. Additionally or alternatively, the point ahead offset isselected such that Tx pointing loss is below a threshold value and thepoint ahead offset is selected such that Rx coupling loss is below thesame threshold value or a different threshold value.

The point ahead offset may be smaller than the point ahead and it maychange over time. The direction of the Rx and Tx pathways may be equalto the point ahead minus the point ahead offset. In some embodiments,the point ahead is less than 20 microradians and the point ahead offsetis greater than zero and less than or equal to 9 microradians.

The Rx pathway may include a wavefront sensor that receives a portion ofthe Rx optical beam. In this case, the control system may modify thepoint ahead offset based on a location or position of the portion of theRx optical beam on the wavefront sensor. In some embodiments, the Rxpathway includes a single mode fiber oriented to receive at least aportion of the Rx optical beam. Additionally or alternatively, the Rxpathway may include an optical amplifier configured to amplify at leasta portion of the Rx optical beam.

Other aspects include components, devices, systems, improvements,methods, processes, applications, computer readable mediums, and othertechnologies related to any of the above.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the disclosure have other advantages and features whichwill be more readily apparent from the following detailed descriptionand the appended claims, when taken in conjunction with the examples inthe accompanying drawings, in which:

FIG. 1A is a block diagram of a free space optical (FSO) node, accordingto one embodiment.

FIG. 1B is a block diagram of a beam steering unit of the FSO node,according to one embodiment.

FIGS. 2A and 2B are diagrams of an optical tracking device (the Tx/RxSubassembly), according to one embodiment.

FIGS. 3A-3E are diagrams of an FSO link between two satellites,according to some embodiments.

FIGS. 4A-4D are graphs of loss functions, according to some embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The figures and the following description relate to preferredembodiments by way of illustration only. It should be noted that fromthe following discussion, alternative embodiments of the structures andmethods disclosed herein will be readily recognized as viablealternatives that may be employed without departing from the principlesof what is claimed.

FIG. 1A is a block diagram of an FSO node 100, according to oneembodiment. The FSO node 100 communicates with a remote FSO node ofsimilar design by transmitting and receiving FSO optical signals 120(also referred to as beams). The FSO node 100 includes a fore optic 102(also referred to as a telescope), Tx/Rx subassembly 104, control system110, optical circulator 114, Tx source 116, amplifier 122, and Rxelectronics 118. The fore optic 102 is optically coupled to the Tx/Rxsubassembly 104. The Tx/Rx subassembly 104 is optically coupled to theRx electronics 118 and to the Tx source 116 by the optical circulator114. The Rx electronics 118, Tx source 116, Tx/Rx subassembly 104, andfore optic 102 are electrically coupled to the control system 110. Inother embodiments, the FSO node 100 may contain additional, fewer, ordifferent components.

FIG. 1A illustrates a node design with collinear Tx and Rx opticalcomponents. This eliminates the “boresight problem” of having tomaintain alignment between Tx and Rx optical systems. For example, thealignment between the Tx and Rx optical systems remains constant duringoperation of the node (e.g., the collinear system doesn't include acomponent configured to change the alignment). In the collinearapproach, the same primary optical system carries signals both for Txand Rx paths, reducing weight and complexity of separate opticalcomponents for Tx and Rx beams. Collinear optical design also simplifiesthe system and enhances reliability by eliminating separate pointing andtracking mechanisms for Tx and Rx. Reliance on fiber-based components(as in FIG. 1A) further reduces system weight and simplifies assembly.These properties are especially important for systems to be used aboardspacecraft or other applications that require robustness, light weight,and high reliability.

The light 120 includes received and transmitted optical signals for FSOnode 100. The received and transmitted optical signals can includedata-encoded communication information. As indicated in FIG. 1A, opticalsignals 120 travel bi-directionally into (i.e., the receive direction)and out of (i.e., the transmit direction) the FSO node 100 through thefore optic 102.

The fore optic 102 is an optical component that collects and directsreceived light signals 120 to the Tx/Rx subassembly 104 and directstransmit light signals 120 from the Tx/Rx subassembly 104 to the remoteFSO node 100. For instance, the fore optic 102 may include a lens and/ora beam expander. The position of the fore optic 102 may be physicallycontrolled by the control system 110. For example, based on positioninformation from the Tx/Rx subassembly, the control system 110 mayadjust the position of the fore optic 102 (or a component of the foreoptic) such that the received light 120 is centered upon the Tx/Rxsubassembly 104. The fore optic 102 may be configured to spread, focus,redirect, and otherwise modify the light 120 passing through the foreoptic 102. The fore optic 102 may also include optical componentsconfigured to reduce external effects not relevant to beam alignment ofthe received light 120. For example, the fore optic 102 may include acomponent that reduces atmospheric scintillation effects. The fore optic102 may be as simple as a single lens or it may include additionaloptical components, such as diffusers, phase screens, beam expanders,and lenses.

In some embodiments, the fore optic 102 includes a beam steering unit(BSU). FIG. 1B illustrates an example fore optic 102 with a BSU 124. Forsimplicity, the BSU 124 is the only illustrated component of the foreoptic 102 in FIG. 1B. However, as previously described, the fore optic102 may include additional or other components.

The BSU 124 is an optical component that receives instructions from thecontrol system 110 to direct Rx beams 120 a to the Tx/Rx subassembly 104and directs Tx beams 120 b to a remote FSO node. The BSU 124 may takemany different forms. The BSU 124 can be a mechanically drivenreflective or refractive device. Examples of such devices includemirrors, Fresnel devices, lenslet arrays and more. A mechanical driverfor any one of these examples can include voice-coil actuators,piezoelectric actuators, servo-motor driven positioners, and many otherapproaches. Microelectronic arrays (MEMS) devices can also be used tosteer a beam. Opto-acoustic devices that exploit acoustic waves inreflective or refractive materials can also be used.

The Tx direction may be determined or updated based on feedback signals(e.g., alignment errors) from the control system 110. In some cases, theBSU 124 is oriented to direct the Tx beam 120 b along the same directionas the Rx beam 120 a is received (e.g., if the FSO nodes are stationaryrelative to each other). In other cases, the BSU 124 is oriented todirect the Tx beam 120 b along a different direction. As illustrated inFIG. 1B, in these cases, the Tx/Rx subassembly 104 may receive the Rxbeam at a different angle or location than the Tx beam. In an examplecase, atmospheric conditions between nodes 100 can affect beamsdifferently depending on their propagation direction. In this case, Txand Rx beams may travel different optical paths between nodes 100. Inanother example case, if a remote node is moving, the BSU 124 may directa Tx beam with an angular bias to account for travel time of the Txbeam.

Referring back to FIG. 1A, the Tx/Rx subassembly 104 receives lightsignals 120 directed by the fore optic 102 and transmits light signals120 from the Tx source 116. The Tx/Rx subassembly 104 is used forpositioning, acquisition, and tracking (PAT). Specifically, light 120received by the Tx/Rx subassembly 104 is used to determine alignmentinformation of the Tx/Rx subassembly 104 in relation to the fore optic102 and the FSO node 100 in relation to a remote FSO node. For example,the Tx/Rx subassembly 104 includes detection sensors that detect thelight signals 120 from the fore optic 102 and determine alignmentinformation from the received light 120. The Tx/Rx subassembly 104 canprovide alignment information to the control system 110.

The optical circulator 114 allows light signals 120 to pass between theRx electronics 118, Tx source 116, and Tx/Rx subassembly 104. Throughthe circulator 114, received light signals 120 from the Tx/Rxsubassembly 104 are directed to the Rx electronics 118 and transmitlight signals 120 from the Tx source 116 are directed to the Tx/Rxsubassembly 104. The circulator 114 can be a single-mode or multi-modecirculator.

The Tx/Rx fiber 112 is an optical fiber, such as a single-mode fiber(SMF) or a multi-mode fiber (MMF), that carries both Rx and Tx beams.The use of a single-mode fiber may allow higher bandwidths compared to amulti-mode fiber. For example, for bandwidths of 10 Gbps (gigabytes persecond) and higher, the Rx electronics 118 require a single mode fiber.In another example, long range FSO link systems may also require SMFreceivers to get higher receiver sensitivity. In FSO Rx subsystems thatuse a single-mode fiber to collect the Rx optical beam, the core of theoptical fiber is small (e.g., a diameter of 9 mm) and Rx signal thatmisses it is lost. Therefore, even a small pointing misalignment causessignificant coupling loss. This may motivate the use of a multi-modefiber or a plurality of single-mode fibers in a receiver system. Thesecollect the optical beam over a wider range of incoming angles, andthereby confer additional robustness in pointing and tracking. However,some Rx signal is nevertheless lost whenever pointing ahead isimplemented. Furthermore, the use of a multi-mode fiber or multiplesingle-mode fibers may add undesirable system complexity and cost. WhileFIG. 1A illustrates a single Tx/Rx fiber 112, the node 100 may includemultiple fibers, for example, a Tx fiber and a Rx fiber that areparallel to each other.

The Rx electronics 118 determine data encoded in the received light 120signals. The Rx electronics 118 can include optical components, such asoptical filters, to prepare the received light 120 to be converted toelectrical signals. The Rx electronics 118 can include photodetectors,such as avalanche photodiodes (APDs), which convert the received light120 into an electrical signal. The photodetectors may be capable ofdetecting light 120 in low light and high light situations (e.g., highsaturation). The Rx electronics 118 can also include any furtherelectronics and/or computer instructions that process the electricalsignal corresponding to the received light 120, which may be embodied indigital or analog circuits, implemented using any one or more ofapplication specific integrated circuits (ASICs), field-programmablegate arrays (FPGAs), and general purpose computing circuits, along withcorresponding memories and computer program instructions for carryingout operations on the data. The specifics of these components are notshown for clarity and compactness of description.

In some embodiments, the FSO node 100 includes an optical amplifier 122.Due to the high data-rates (e.g., 10 Gbps or higher) and distancesbetween FSO nodes, the received Rx signals may be weak. Thus, an opticalamplifier may amplify Rx signals in the fiber 112 to levels suitable forelectronic detection and data decoding. Some example optical amplifiersrequire coupling to a single mode fiber and cannot couple to amulti-mode fiber.

The Tx source 116 converts transmit data into transmit light signals120. The transmitted light 120 is sent to the Tx/Rx subassembly 104 viathe circulator 114. The Tx source 116 can include a laser and associatedoptical components to produce the transmit light 120 signals. Forexample, the Tx source 116 includes a seed laser and one or more fiberamplifiers, such as an Erbium-doped fiber amplifier (EDFA). The Txsource 116 can also include electronics and/or computer instructionsthat modulate the transmit light signals 120 and encode the underlyingdata to be transmitted, including any other physical (PHY) layer ormedium access control (MAC) related processes, such as the addition oferror correction and so on. Similar to the Rx electronics 118, the Txsource 116 may be embodied in digital or analog circuits, implementedusing any one or more of application specific integrated circuits(ASICs), field-programmable gate arrays (FPGAs), and general purposecomputing circuits, along with corresponding memories and computerprogram instructions for carrying out operations on the data. Thespecifics of these components are not shown for clarity and compactnessof description.

The control system 110 includes software or hardware that receives inputfrom the Rx electronics 118 and the Tx/Rx subassembly 104 to control aposition of the fore optic 102 (or a component of the fore optic 120)and the Tx/Rx subassembly 104 such that the fore optic 102 directsreceived light signals 120 to the Tx/Rx subassembly 104. Furthermore,the control system 110 can send instructions to position the fore optic102 and the Tx/Rx subassembly 104 such that transmit light signals 120are transmitted to a remote FSO node 100. For example, the controlsystem 110 may position the fore optic 102 so that a transmit lightsignal 120 propagates along a direction that is approximately parallel(but in an opposition direction) to a received light signal 120. Inanother example, if the remote FSO node is moving, the control system110 can direct a transmit light signal 120 with an angular bias toaccount for the travel time of the transmit light signal 120 and otherdelays due, for example, to received signal processing.

FIGS. 2A and 2B are diagrams of an optical tracking device (the Tx/RxSubassembly), according to one embodiment. Specifically, FIG. 2A is afront view of the Tx/Rx subassembly 104 and FIG. 2B is a perspectiveview of the Tx/Rx subassembly 104. These two figures have similarreference numerals and are discussed together. The Tx/Rx subassembly 104includes an initial end of the Tx/Rx fiber 112 surrounded by a wavefrontsensor 202 in plane 212. The Tx/Rx subassembly 104 receives an exampleRx beam 206. In other embodiments, FIGS. 2A and 2B may containadditional, fewer, or different components.

The example Rx beam 206 is directed by the fore optic 102, travels toplane 212, and contains encoded communication information from a remoteFSO node 100. The Tx/Rx fiber 112 receives a first portion of the Rxbeam 206 a and the wavefront sensor 202 receives a second portion of theRx beam 206 b. If no portion of the Rx beam 206 is received by the Tx/Rxfiber 112 (e.g., Rx beam 206 isn't incident on the fiber or the angle ofincidence is large), then the FSO node 100 is not capturing the dataintended for receipt. In this case, the Tx/Rx subassembly 104 or foreoptic 102 may be repositioned.

The wavefront sensor 202 lies in plane 212 and is used for signalacquisition and alignment of the FSO node 100 system. To do this, thewavefront sensor 202 uses detectors 204 to detect the position of thereceived second portion of the Rx Beam 206 b on the Tx/Rx subassembly104. Based on the detected position of the received second portion ofthe Rx beam 206 b, the FSO node 100 can align optical components of theFSO node 100 (e.g., the fore optic 102) so that the Rx beam 206 iscentered on the wavefront sensor 202 (or another location of the sensor202). In other embodiments, components other than a wavefront sensor mayperform position detection, acquisition, and tracking. By way ofexample, a sensor can be an overmoded guided-wave structure with a meansto measure the power in the structure's propagating modes.

The example wavefront sensor 202 is a quad cell detector. Together, thefour detectors 204 determine the horizontal and vertical position of theRx beam 206 on the wavefront sensor 202. If the displacement of the Rxbeam 206 on the wavefront sensor 202 is not zero, the FSO node 100 mayposition the Tx/Rx subassembly 104, the fore optic 102, or the FSO node100 to reduce or eliminate the displacement. The displacement may bedetermined or calculated based on a comparison of the detected signalsfrom each detector 204. For example, the ratio of the difference of theRx beam 206 on each half of the wavefront sensor 202 divided by thewhole is used to determine a percentage offset from the center of thewavefront sensor 202. The wavefront sensor 202 may include more, fewer,or different detectors 204.

Referring now to FIGS. 3A-3E, two satellites 301, 302 are in orbit(e.g., in low-Earth orbit (LEO)). Each satellite includes an FSO nodeand communicate with each other over an FSO link (e.g., an opticalintersatellite link (OISL)). FIGS. 3A-3E illustrate the FSO link fromthe point of view of the FSO node in satellite 301. A similar situationexists for satellite 302. The first satellite 301 communicates with thesecond satellite 302, and the second satellite 302 moves on a projectedpath 307 (also referred to as known travel path) from location 308R tolocation 308T. FIGS. 3A-3E illustrate the position of satellite 301 attime t1 and the positions of satellite 302 at times t1-Δ, t1, and t1+Δ,where Δ is the travel time for an FSO beam to travel from one satelliteto the other. Since the projected path 307 may be known, satellite 301knows that satellite 302 will be at location 308T when it receives a newbeam transmitted from satellite 301 at time t1. Note that the FSO beamsare illustrated in FIGS. 3A-3E as lines for convenience. Practically,the beams have widths and spread out over time. For example, the beamshave gaussian profiles.

Both satellites being in motion, the Rx beam 303 received by satellite301 at time t1 appears to come from location 308R from second satellite302. Location 308R may be satellite 302's actual location at the time oftransmitting Rx beam 303. However, location 308R may not be satellite302's actual location at time of transmitting Rx beam 303 (discussedfurther with respect to FIG. 3E).

The point ahead angle in this system is angle 310. As previouslydescribed, a point ahead angle is an angular bias relative to a Rxdirection that accounts for travel time of a Tx beam (for situationswhere the relative velocity and distance between a local and a remoteFSO node is large). In FIG. 3A, the point ahead angle 310 is equal tothe sum of Rx beam misalignment angle 305 and the Tx beam mispointingangle 306. The Rx misalignment angle 305 is the angular deviation of theTx beam 304 away from the Rx direction, and the Tx beam mispointingangle 306 is the angular deviation of the Tx beam 304 away from thepoint ahead angle 306.

To maintain the FSO link, satellite 301 aims to transmit Tx beam 304 sothat it is received by satellite 302 and also aims to receive Rx beam303 from satellite 302. However, satellite 301 may include collinear Txand Rx optical components (e.g., as described with reference to FIG.1A). In this case, if satellite 301 points towards location 308T (e.g.,see FIG. 3C), Tx beam 302 will be received by satellite 302 with littleor no coupling losses (“Tx pointing losses”) (this assumes thatsatellite 302 will point at satellite 302 to receive Tx beam 302).However, the Rx optical path of satellite 301 will be misaligned withthe propagation direction of Rx beam 303 because satellite 301 is notpointing at location 308R, resulting in increased Rx coupling losses. Insome cases, this situation results in satellite 301 receiving none ofthe Rx beam 303 or receiving an amount of Rx beam 303 that isinsufficient to maintain the FSO link.

Alternatively, if satellite 301 points towards location 308R (e.g., seeFIG. 3B), Rx beam 303 will be received by satellite 301 with little orno Rx coupling losses (this assumes that satellite 302 pointed atsatellite 301 to transmit Rx beam 303). However, the Tx optical path ofsatellite 301 will not be aligned with location 308T, resulting inincreased Tx pointing losses. In some cases, this situation results insatellite 302 receiving none of the Tx beam 304 or receiving an amountof Tx beam 304 that is insufficient to maintain the FSO link.

Since satellite 301 cannot point in two directions at once, the actualTx beam 304 direction is determined by balancing the tradeoff betweenthese two situations. By pointing satellite 301 somewhere betweenlocations 308R and 308T, satellite 301 may receive enough of Rx beam 303and satellite 302 may receive enough of Tx beam 304 to maintain the FSOlink. The specific Tx direction is determined by modifying the pointahead angle by a point ahead offset angle. This modified point aheadangle reduces Rx coupling losses (compared to the point ahead angle)while keeping Tx pointing losses low enough to maintain the opticallink. For example, the modified point ahead angle (e.g., angle 305) ofTx beam 304 is the point ahead angle 310 minus the point ahead offsetangle (e.g., angle 306). In this case, as the point ahead offset angleis increased, the Rx coupling losses decrease and the Tx pointing lossesincrease.

Referring to FIG. 3E, satellite 302 may also be pointing at modifiedpoint ahead angles. In this case, the actual Rx beam 308 may notpropagate directly toward satellite 301 (which was the ideal point aheaddirection for satellite 302 at time t1-Δ), but instead may propagate ata modified angle (indicated by the direction of beam 308). Furthermore,instead of satellite 302 pointing along the ideal point ahead directionfor satellite 301, satellite 302 may point at another modified angle(indicated by the dotted line at location 308T). These modified anglesby satellite 302 may cause additional Rx coupling losses and Tx pointinglosses for satellite 301. In some embodiments, satellite 301 accountsfor the modified angles of satellite 302 when determining the modifiedpoint ahead angle. In other embodiments, satellite 301 may only considerthe ideal point ahead directions.

Referring back to the point ahead offset, the point ahead offset anglemay be chosen so that a sum of the Rx coupling losses and the Txpointing losses is reduced (e.g., minimized). In another example, thepoint ahead offset angle is chosen so that the Rx coupling losses andthe Tx pointing losses are below a threshold, where the thresholdensures that enough of each beam is received to maintain the FSO link.Additionally or alternatively, the point ahead offset angle may dependon the FSO link distance (e.g., derived through timing data exchange),the point ahead angle, the Tx and Rx beam divergences, the configurationof the Tx/Rx subassembly 104, the fiber type of the Tx/Rx fiber 112,aperture size of the telescope 102, or the jitter magnitude.

In some embodiments, the point ahead offset angle ranges from 1-9microradians (urad). The modified point ahead angle may be implementedwhen the point ahead angle is 20 urad or less. If the point ahead angleis larger than 20 urad, there may not be a point ahead offset angle thatresults in sufficient Rx coupling and Tx pointing.

The point ahead offset may be a function of the point ahead angle. Thus,the point ahead offset angle may change over time as the point aheadangle changes (e.g., due to changes in relative speed or distancebetween satellites 301 and 302). For example, if satellites 301 and 302are in different orbits or in orbits with significant ellipticity, theymay have point ahead angles that change over time due their changingrelative heading, speed, and distance. In another example, twosatellites in the same elliptical orbit may experience differentaccelerations in different parts of the orbit and therefore may havevariable point-ahead, even though they nominally follow identical pathsaround the Earth. For example, satellites in identical orbits witheccentricity 0.7 but separated by ¼ period sometimes see each otherapproaching or receding to either side of orbital perigee. However, insome situations, the point ahead angle may be constant. For example, ifsatellites 301 and 302 are in orbits with fixed relative speed andposition (e.g., identical but phase-offset circular orbits around theEarth), such orbits may result in a constant point ahead angle.

The modified point ahead, point ahead, or point ahead offset may bepre-calculated and stored in a lookup table that is accessed duringoperation of the FSO node. For example, modified point ahead angles maybe predetermined for FSO nodes in orbit since their absolute andrelative motions may follow well-defined orbital trajectories. Forexample, if satellites 301 and 302 are in identical orbits witheccentricity 0.7 but separated by ¼ period, the two satellites may havepoint-ahead angles that switch positive and negative but predictablyrepeat with each orbit. In another example, satellites 301 and 302 havetwo circular orbits that are inclined relative to each other. Suchorbits cross each other and thus may predictably switch pointing angleand point-ahead polarity at the crossings. Other embodiments may computethe modified point ahead angles dynamically during operation (e.g., atthe time it is required to orient the Tx beam 304). For example, apoint-ahead processor (e.g., implemented as part of control system 110)may use a priori knowledge of orbital parameters to determineinstantaneous velocities and directions between the nodes and therebyderive a modified point ahead angle. Time and frequency synchronizationbetween FSO nodes may also be available to provide an additionalestimate of relative range. This can be used to verify or refine theorbital parameters and the modified point ahead direction.

In some embodiments, the modified point ahead is based on real timevalues, such as the received power, signal-to-noise ratio, or bit errorrates at one or both FSO nodes. In some embodiments, a modified pointahead is determined based on signal amplitude or signal quality metricsas they are received or derived at an FSO node. Other example real timevalues include beam amplitude, signal-to-noise ratio, and polarizationbalance. After communication is established between a remote and localnode, an embodiment may transfer this information over the optical linkso that both node controllers maintain history of signal strength atboth nodes. This enables the processors at either or both nodes tojointly and cooperatively optimize the modified point ahead offsetangles in either or both directions.

In one approach, a modified point ahead angle is implemented by adding awavefront offset to the feedback loop from a wavefront sensor (e.g.,sensor 202). For example, to maintain the modified point ahead angle,the control system 110 instructs the beam steering unit 124 to steer anRx beam at a location on the wavefront sensor that is offset from thecenter.

The modified point ahead angle may be implemented even when the opticalbeams between enter and leaves Earth's atmosphere (e.g., if one node isin low-Earth orbit (LEO) and the other node is on earth's surface). Inthese situations, point ahead functionality can be extended byincorporating a refraction computation, based on orbital mechanics or onpointing aberrations that can be attributed to the atmosphere.

FIG. 4A is a graph of an example Tx pointing loss function thatdescribes Tx pointing losses. Specifically, FIG. 4A illustrates Txpointing loss vs. pointing error for an FSO node with a 90-millimeteraperture, a beam divergence of 37.64 urad, and it assumes 6 urad ofjitter, according to an embodiment. Pointing error is the deviation awayfrom the point ahead angle (with respect to FIG. 3A, the pointing erroris the Tx mispointing angle 306). Notice that as pointing errorincreases, the pointing loss exponentially increases. In other words,the larger the point ahead offset angle, the less a remote FSO node willreceive of a Tx beam. Practically, beam divergence reduces the fractionof power transmitted from one node that is received by the other, andthis pointing loss increases with range. Thus, for communication betweennodes at short range, the tolerable pointing loss may be greater than atlonger ranges. In some embodiments, a maximum tolerable pointing lossmay depend on the FSO link error margin which may be a function of manyparameters e.g., including link distance.

FIG. 4B is a graph of example Rx coupling loss functions that describeRx coupling losses. Specifically, FIG. 4B describes Rx coupling loss vs.Rx angle for example lenses (in the fore optic) with various f-numbers(F #), according to an embodiment. The graph describes an FSO systemwith a 90 mm aperture. Rx angle is the deviation away from the Rxdirection of a Rx beam (with respect to FIG. 3A, the Rx angle deviationis the Rx beam misalignment angle 305), and f-number refers to the ratioof focal length to the diameter of the entrance pupil of the lens.

Similar to the previous figure, the plots in FIG. 4B are exponential.Thus, as Rx angle increases, the coupling losses increase at a fasterrate. Additionally, for small Rx angles (e.g., less than 10 urad),lenses with smaller f-numbers have larger coupling losses, while forlarger Rx angles (e.g., larger than 10 urad), lenses with smallerf-numbers have smaller coupling losses. As described above with respectto tolerable pointing losses, tolerable coupling losses may also dependon the FSO link configuration. For example, for communication betweennodes at short range, the tolerable coupling loss may be greater than atlonger ranges.

FIG. 4C is another graph of example Rx coupling loss functions thatdescribe Rx coupling losses. Specifically, FIG. 4C is a graph ofcoupling loss vs. airy disk mismatch for lenses (in the fore optic) withvarious f-numbers, according to an embodiment. The airy mismatchdescribes the difference between the location of the focus of an Rx beamand a center point of the Rx fiber. In the example of FIG. 2A, the airymismatch is the distance between a center point of the fiber 112 and acenter point of Rx beam 206. Similar to the previous figures, the plotsin FIG. 4C are exponential. Thus, as airy disk mismatch increases, thecoupling losses increase at a faster rate. Additionally, lenses withsmaller f-numbers have larger coupling losses.

FIG. 4D is a graph of loss vs. modified point ahead angle for variouspoint ahead angles for a fore optic lens with an f-number of 4.1. Lossdescribes the combined Rx coupling losses and the Tx pointing losses fora given modified point ahead angle. Thus, the point ahead offset anglemay be determined by subtracting the modified point ahead angle from thepoint ahead angle. Since Rx coupling losses and Tx pointing losses areeach exponential, the resulting combined loss plots have u-shapes. Thus,for a given plot, the optimal modified point ahead angle is an angle atthe bottom of the u-shape that minimizes the losses. In the example ofFIG. 4D, modified point ahead angles of 5, 6.5, 8.8, and 10.2 uradrespectively minimize the loss for point ahead angles of 5.7, 10.5,15.4, and 19.5 urad. Note that as point ahead angle increases the plotshave higher losses and the optimal modified point ahead angle increases.

Although the detailed description contains many specifics, these shouldnot be construed as limiting the scope of the invention but merely asillustrating different examples. It should be appreciated that the scopeof the disclosure includes other embodiments not discussed in detailabove. For example, similar conditions and considerations of point-aheadoffsets pertain for communication between ground-based stations andstations on aircraft or in low orbit. Similar considerations may alsopertain for adversarial uses, as for laser weapons and for communicationjamming devices. Various other modifications, changes and variationswhich will be apparent to those skilled in the art may be made in thearrangement, operation and details of the method and apparatus disclosedherein without departing from the spirit and scope as defined in theappended claims. Therefore, the scope of the invention should bedetermined by the appended claims and their legal equivalents.

In the claims, reference to an element in the singular is not intendedto mean “one and only one” unless explicitly stated, but rather is meantto mean “one or more.” In addition, it is not necessary for a device ormethod to address every problem that is solvable by differentembodiments of the invention in order to be encompassed by the claims.Further, unless expressly stated to the contrary, “or” refers to aninclusive or and not to an exclusive or. For example, a condition A or Bis satisfied by any one of the following: A is true (or present) and Bis false (or not present), A is false (or not present) and B is true (orpresent), and both A and B are true (or present).

Alternate embodiments may be implemented using computer hardware,firmware, software, and/or combinations thereof. Implementations can beimplemented in a computer program product tangibly embodied in acomputer-readable storage device for execution by a programmableprocessor; and method steps can be performed by a programmable processorexecuting a program of instructions to perform functions by operating oninput data and generating output. Embodiments can be implementedadvantageously in one or more computer programs that are executable on aprogrammable computer system including at least one programmableprocessor coupled to receive data and instructions from, and to transmitdata and instructions to, a data storage system, at least one inputdevice, and at least one output device. Each computer program can beimplemented in a high-level procedural or object-oriented programminglanguage, or in assembly or machine language if desired; and in anycase, the language can be a compiled or interpreted language. Suitableprocessors include, by way of example, both general and special purposemicroprocessors. Generally, a processor will receive instructions anddata from a read-only memory and/or a random access memory. Generally, acomputer will include one or more mass storage devices for storing datafiles; such devices include magnetic disks, such as internal hard disksand removable disks; magneto-optical disks; and optical disks. Storagedevices suitable for tangibly embodying computer program instructionsand data include all forms of non-volatile memory, including by way ofexample semiconductor memory devices, such as EPROM, EEPROM, and flashmemory devices; magnetic disks such as internal hard disks and removabledisks; magneto-optical disks; and CD-ROM disks. Any of the foregoing canbe supplemented by, or incorporated in, ASICs (application-specificintegrated circuits), FPGAs and other forms of hardware.

Depending on the form of the components, the “coupling” betweencomponents also take different forms. Dedicated circuitry can be coupledto each other by hardwiring or by accessing a common register or memorylocation, for example. Software “coupling” can occur by any number ofways to pass information between software components (or betweensoftware and hardware, if that is the case). The term “coupling” ismeant to include all of these and is not meant to be limited to ahardwired permanent connection between two components. In addition,there may be intervening elements. For example, when two elements aredescribed as being coupled to each other, this does not imply that theelements are directly coupled to each other nor does it preclude the useof other elements between the two.

What is claimed is:
 1. A free space optical (FSO) communication terminalcomprising: a transmit (Tx) pathway configured to transmit adata-encoded Tx optical beam through free space to a remote FSOcommunication terminal, wherein the Tx pathway is oriented along adirection in free space, and wherein a relative motion between the twoFSO communication terminals is known; a receive (Rx) pathway configuredto receive a data-encoded Rx optical beam transmitted through free spacefrom the remote FSO communication terminal, wherein the Rx pathway isoriented along the same direction in free space as the Tx pathway; and acontrol system configured to: determine a pointing direction for the Rxand Tx pathways, wherein the pointing direction is offset relative to apoint ahead and is based on both (a) estimated reception of the Txoptical beam at the remote FSO communication terminal, given the knownrelative motion, and (b) estimated reception of the Rx optical beam atthe FSO communication terminal, given the known relative motion; andcontrol a beam steering unit to adjust the orientation of the Rx and Txpathways in free space based on the determined pointing direction. 2.The FSO communication terminal of claim 1, wherein the estimatedreception of the Tx optical beam at the remote FSO communicationterminal is based on a Tx pointing loss function.
 3. The FSOcommunication terminal of claim 2, wherein the estimated reception ofthe Rx optical beam at the FSO communication terminal is based on a Rxcoupling loss function.
 4. The FSO communication terminal of claim 3,wherein the determined pointing direction for the Rx and Tx pathwaysminimizes a sum of the Tx pointing loss function and the Rx couplingloss function.
 5. The FSO communication terminal of claim 3, wherein thedetermined pointing direction for the Rx and Tx pathways reduces a Txpointing loss below a threshold value.
 6. The FSO communication terminalof claim 3, wherein the determined pointing direction for the Rx and Txpathways reduces a Rx coupling loss below a threshold value.
 7. The FSOcommunication terminal of claim 1, wherein the offset is smaller thanthe point ahead.
 8. The FSO communication terminal of claim 7, whereinthe adjusted orientation of the Rx and Tx pathways is equal to the pointahead minus the offset.
 9. The FSO communication terminal of claim 7,wherein the point ahead is less than 20 microradians.
 10. The FSOcommunication terminal of claim 1, wherein the Rx pathway remainsoriented along the same direction as the Tx pathway during operation ofthe FSO communication terminal.
 11. The FSO communication terminal ofclaim 1, wherein the FSO communication terminal is co-borsighted. 12.The FSO communication terminal of claim 1, wherein the Rx pathwayincludes a wavefront sensor that receives a portion of the Rx opticalbeam.
 13. The FSO communication terminal of claim 12, wherein thecontrol system is configured to modify the offset based on a location ofthe portion of the Rx optical beam on the wavefront sensor.
 14. The FSOcommunication terminal of claim 1, wherein the Rx pathway includes asingle mode fiber oriented to receive at least a portion of the Rxoptical beam.
 15. The FSO communication terminal of claim 1, wherein theRx pathway includes an optical amplifier configured to amplify at leasta portion of the Rx optical beam.
 16. The FSO communication terminal ofclaim 1, wherein the offset is greater than zero and less than or equalto 9 microradians.
 17. The FSO communication terminal of claim 1,wherein the control system is configured to change the offset over time.18. The FSO communication terminal of claim 1, wherein the remote FSOcommunication terminal is mounted to a satellite in orbit.
 19. The FSOcommunication terminal of claim 1, wherein: the poitning direction forthe Rx and Tx pathway is stored in a lookup table; and to determine thepointing direction for the Rx and Tx pathways, the control system isconfigured to access the pointing direction in the lookup table.
 20. TheFSO communication terminal of claim 1, wherein to determine the pointingdirection for the Rx and Tx pathways, the control system is configuredto dynamically compute the pointing direction during operation of theFSO communication terminal.
 21. A free space optical (FSO) communicationterminal comprising: a transmit (Tx) pathway means configured totransmit a data-encoded Tx optical beam through free space to a remoteFSO communication terminal, wherein the Tx pathway means is orientedalong a direction in free space, and wherein a relative motion betweenthe two FSO communication terminals is known; a receive (Rx) pathwaymeans configured to receive a data-encoded Rx optical beam transmittedthrough free space from the remote FSO communication terminal, whereinthe Rx pathway means is oriented along the same direction in free spaceas the Tx pathway means; and a control system means configured to:determine a pointing direction for the Rx and Tx pathway means, whereinthe pointing direction is offset relative to a point ahead and is basedon both (a) estimated reception of the Tx optical beam at the remote FSOcommunication terminal, given the known relative motion, and (b)estimated reception of the Rx optical beam at the FSO communicationterminal, given the known relative motion; and control a beam steeringmeans to adjust the orientation of the Rx and Tx pathway means in freespace based on the determined pointing direction.
 22. The FSOcommunication terminal of claim 21, wherein the offset is smaller thanthe point ahead.